Signal generation apparatus and method for seafloor electromagnetic exploration

ABSTRACT

An apparatus for generating electronic signals for application in subsea electromagnetic exploration. A surface generated high-voltage low-current source signal stabilized at a first frequency is supplied to a deep-tow vehicle ( 18 ) via an umbilical cable ( 16 ). The high-voltage low-current signal is transformed at the deep-tow vehicle to a high-current low-voltage a.c. signal by a transformer ( 52 ) within a cycloconverter ( 30 ). A semiconductor relay bridge ( 104 ) provides switchable rectification of the high-current low-voltage a.c. signal to provide a quasi-square wave at a second frequency, lower than the first frequency, for supply to a transmitting antenna ( 22 ) towed by the deep-tow vehicle. The times of the rectification switching are dependent on zero crossings of the high-current low-voltage a.c. signal. Allowable rectification switching times may be gated to occur only within pre-determined time windows to avoid noise-induced zero-crossing switching. The apparatus allows multiple transmission frequencies to be derived from a single stabilized source and improves spectral integrity by avoiding rectification switching at zero-crossings not occurring at the first frequency.

This application is a national phase of International Application No.PCT/GB02/04460 filed Oct. 2, 2002 and published in the English language.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus and method for generatingelectronic signals for use in the field of seafloor electromagneticexploration.

Determining the response of the sub-surface strata within the earth'scrust to electromagnetic fields is a valuable tool in the field ofgeophysical research. The geological processes occurring in thermally,hydrothermally or magmatically active regions can be studied, forexample. In addition, electromagnetic sounding techniques can providevaluable insights in to the nature, and particularly the likelyhydrocarbon content, of subterranean reservoirs in the context ofsubterranean oil exploration and surveying.

Seismic techniques are often used during oil-exploration expeditions toidentify the existence, location and extent of reservoirs insubterranean rock strata. Whilst seismic surveying is able to identifysuch voids the technique is often unable to distinguish between thedifferent possible contents within them, especially for void contentswhich have similar mechanical properties. In the field of oilexploration, it is necessary to determine whether a previouslyidentified reservoir contains oil or just seawater. To do this, anexploratory well is drilled to sample the contents of the reservoir.However, this is an expensive process, and one which provides noguarantee of reward.

Whilst oil-filled and water-filled reservoirs are mechanically similar,they do possess significantly different electrical properties and theseprovide for the possibility of electromagnetic based discriminationtesting. A known technique for electromagnetic probing of subterraneanrock strata is the passive magneto-telluric (MT) method. The signalmeasured by a surface-based electromagnetic detector in response to EMfields generated naturally, such as within the earth's atmosphere, canprovide details about the surrounding subterranean rock strata. Inpractice a series of detectors are used to isolate effects which arelocal to each detector. However, for deep-sea surveys, all but those MTsignals with periods corresponding to several cycles per hour arescreened from the seafloor by the highly conductive seawater. Whilst thelong wavelength signals which do penetrate to the sea-floor can be usedfor large scale undersea probing, they do not provide sufficient spatialresolution to examine the electrical properties of the typicallyrelatively small scale subterranean reservoirs.

To overcome the lack of suitable MT signals at the seafloor, active EMsounding can be employed. Information about the subterranean strata isdetermined by examining the response of remote detectors to anartificial EM source, where both the detectors and source are locatedat, or near, the seafloor. Benefits of this method include the abilityto know a priori the input signal to which the subterranean rock strataare exposed, the ability to select particular frequencies and coherencelengths of EM signal and the ability to set the relative geometry oftransmitter and receiver antennae.

FIG. 1 of the accompanying drawing shows schematically how a surfacevessel 14 undertakes EM sounding of the subterranean rock strata 8 inwhich a reservoir 12 has already been identified. The surface vessel 14floats on the surface 2 of the sea 4. A deep-sea vessel 18 is attachedto the surface vessel 14 by an umbilical connector 16 which provides anelectrical, optical and mechanical connection between the deep-seavessel 18 and the surface vessel 14. The deep-sea vessel 18 is towed bythe surface vessel 14 such that it remains consistently close to theseafloor 6. This is facilitated by an echo-location package 20 whichrelays information about the height of the deep-sea vessel 18 above theseafloor 6 to the surface vessel 14. The deep-sea vessel 18 receiveselectrical power from the ship's on-board power supply via the umbilical16. A cycloconverter unit 30 generates the chosen waveform to besupplied to a transmitting antenna 22. The antenna 22 is towed by thedeep-sea vessel 18. The antenna 22 broadcasts the EM signal into the sea4, and this results in a component passing through the rock strata 8. Aremote instrument package 26 records the signal received by an antenna24 in response to the transmitted EM signal. If the separation of thetransmitting antenna 22 and the receiving antenna 24 is greater than afew hundred meters, the highly conductive seawater strongly attenuatesthe direct signal between them. The components of the EM signal thathave travelled through the rock strata 8 and the reservoir 12 dominatethe received signal and provide information about the electricalproperties of these regions. At the end of the sounding experiment, aremotely deployable flotation device 28 carries the instrument packageto the surface 2 for recovery and retrieval of data for inversionanalysis.

FIG. 2 of the accompanying drawings schematically shows the deep-seavessel 18 and transmitting antenna 22 in more detail. The umbilical 16is attached to the deep-sea vessel 18 via a swivelable connection 32.The echo-location package 20 and cycloconverter unit 30 are carriedwithin the deep-sea vessel 18. A fin 34 helps to stabilise the deep-seavessel 18 as it is drawn through the seawater 4. The antenna 22 isattached to the deep-sea vessel 18 by a towing bar 36. The antenna 22comprises a fore electrode 38 and an aft electrode 42. The EM signalgenerated by the cycloconverter unit 30 is applied to the electrodes 38,42 via signal cables 40, 44. It is the conducting seawater 4 whichprovides the unscreened return path for the electrical current andgenerates a dipolar EM field. The electrodes 38, 42 and cables 40, 44are supported by a neutrally buoyant hose 46. A tail rope 48 supplies adrag force to the trailing end of the antenna 22 to assist in keeping itcorrectly extended.

The exact choice of the waveform supplied to the transmitting antenna 22and the ability to vary its fundamental properties, such as frequency,is important. Different frequencies of EM signal will propagatedifferently through the rock strata 8. Each frequency therefore providesinformation which is sensitive to the particular conditions alongdifferent paths within the rock strata 8, and together allow for moredetailed mapping. The stability of the waveform in amplitude, frequencyand phase are crucial to providing the best possible examination of therock strata 8. For example, with no direct connection between thetransmitting 22 and receiving 24 antennae, it is impossible to transmitinformation about phase drifts in the source EM signal to the instrumentpackage 26. Accordingly, it is impossible to distinguish between a driftin the phase of the source signal and a change in the propagation timebetween source and receiver. The precision to which the electricalproperties of the rock strata, which determine the signal time delay,can be determined is therefore highly dependent on the stability of thesource signal, the generation of which is not a straightforward task.

The requirement to transmit power at a level of several kilowattsthrough the umbilical 16 necessitates the use of a relatively highvoltage, low current supply in order to minimnise transmission losses.However, such an a.c. power source has significantly differentcharacteristics from those desired for the outgoing waveform.

It is the purpose of the cycloconverter unit 30 is to transform theinput a.c. power supply (high voltage, low current, fixed frequencysinusoid) into the desired transmitter waveform (low voltage, highcurrent, variable and controllable frequency and waveform).

One way of generating an output signal of the desired frequency from theinput signal is through a half-wave rectifying bridge circuit that iscontrollably switchable at the zero crossings of the input signal.

FIG. 3 is a graph which schematically represents an ideal output signalfrom such a bridge, which has reduced the frequency of the output signalby a factor of 5 by switching the bridge at every fifth zero crossing ofthe signal at the input frequency.

FIG. 4 of the accompanying drawings schematically shows an ideal 256 Hzinput voltage as a function of time. The switching takes place on zerocrossings of the input waveform (marked with bold vertical lines in thefigure). The control operates by detecting these zero crossings, andimmediately switching the bridge to provide the appropriate polarity ofoutput for the next half cycle. The frequency and phase of thetransmitter output signal depends on the timing at which the polarity ofthe output half cycles is changed. This can be controlled in two ways.

One approach would be to rely on using a frequency stabillised powersupply from the surface vessel to the transmitter's cycloconverter unit.Control over both the frequency and phase of the output signal can inprincipal be achieved by controlling the phase and frequency stabilityof the power supply. However, such an approach faces technical problemscaused by the capacitive and inductive effects in the tow cable, thecycloconverter itself, and the dipole transmitting antenna, as now beingexplained.

The tow cable may be constructed using either co-axial or spiral woundelectrical conductors. In either case, and especially in the case of aco-axial construction tow cable, several kilometres of cable constitutea very significant capacitance between the power source and the deep towvehicle. Typically the cable also has some inductance; but thetransmission characteristics will vary from cable to cable, and to alesser extent will also depend on the relative amounts of the cable thatare immersed in sea water or wound onto the drum of the towing winch.

The transmitting dipole antenna must be designed to have as low aresistance as possible, in order to optimise the transmitter dipolemoment for a given power level. It will however have a significantself-inductance, which will to some extent depend on the characteristicsof the seawater through which it is being towed and its proximity to,and the properties of, the seafloor.

There are two major effects of the capacitive and inductive propertiesof these components of the transmitter system. First, in general thecurrent at any point is not in phase with the voltage. Second, even ifthe power supply at the surface vessel is designed for low harmonicdistortion, the input voltage and current waveforms at the deep-towvessel are significantly affected by higher harmonics of the supplyfrequency and by standing waves set up in the system between the shipboard power supply and the transmitting electrodes in the antenna. Theexact properties of these harmonics and standing waves are difficult topredict, and are likely to vary significantly between installations ondifferent vessels, and even within a single deployment of thetransmitter system as tow cable is paid out and hauled in.

FIG. 5 of the accompanying drawings schematically shows how the ideal256 Hz input voltage indicated in FIG. 4 might more realistically appeardue to the effects described above. In response to the input waveformindicated in the figure, the cycloconverter control is liable to detectspurious and unpredictable zero crossings which are due to harmonics orstanding waves, and not to the fundamental supply waveform. This isapparent from the number of crossings, again marked with bold verticallines in the figure. If the zero crossings are used to control theswitching of individual half sinusoids, and to control the timing ofoutput waveform polarity reversals, then the source becomes subject tounpredictable frequency jitter and phase drift.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided acycloconverter for subsea electromagnetic exploration, comprising:

a transformer having a primary side for receiving a high voltage lowcurrent signal of a first frequency and a secondary side for outputtinga low voltage high current signal also of the first frequency;

a switching circuit connected to receive the low voltage high currentsignal from the secondary side of the transformer and to switch itspolarity so as to generate an output signal having a significantcomponent at a second frequency lower than the first frequency; and

a controller configured to control the switching circuit responsive to ameasurement of the first frequency.

The cycloconverter preferably further comprises a zero-crossing detectorfor measuring zero crossings at the first frequency and supplying azero-crossing signal to the controller, the controller being configuredto supply a switching signal to the switching circuit when azero-crossing signal is received during a time window that has beendetermined by the controller responsive to the measurement of the firstfrequency.

A further aspect of the invention relates to a submersible vehiclefitted with a cycloconverter according to the first aspect of theinvention.

Another aspect of the invention relates to a method of controllingswitching events in a cycloconverter during subsea electromagneticexploration, comprising:

supplying a high power signal to the cycloconverter at a firstfrequency;

obtaining a switching signal by measuring the high power signal;

locking into the first frequency,

predicting a time window for a next desired switching event; and

gating the switching signal with the time window to suppress switchingoutside the time window.

Accordingly the cycloconverter can prevent the switching being initiatedby spurious zero crossings, while retaining the advantages ofcontrolling the outgoing waveform by locking it to the phase andfrequency of the ship-board power supply. The ship-board power supplycan be controlled by a high quality and readily monitored frequencystandard on the towing vessel.

In an embodiment of the invention a secondary tining circuit is providedwithin the cycloconverter, which screens the observed zero crossings andselects only those corresponding to zero crossings of the fundamentalfrequency for output waveform generation. Since the frequency of thefundamental component of the power supply is both known and stable, theinterval between zero crossings of this component is predictable. Thescreening process involves passing the output from the zero crossingdetector—which consists of a series of pulses, some of which arecrossings of the fundamental frequency and some of which are due tohigher harmonics and standing waves—through a gating system. Zerocrossings which occur significantly earlier than the time of the nextpredicted crossing of the fundamental frequency are rejected. A zerocrossing will not be accepted until a time approaching a half period ofthe fundamental has elapsed. Once an acceptable zero crossing hasoccurred, the cycloconverter switches on the semiconductor in the outputbridge appropriate to the next output half-sinusoid polarity, andadvances a counter which keeps track of the overall output waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 is a schematic plan view representation of an exploratory EMsounding survey;

FIG. 2 is a schematic plan view further detailing the submersibledeep-sea vessel shown in FIG. 1;

FIG. 3 is a graph showing an output signal waveform for supplying to anantenna of the vessel;

FIG. 4 is a schematic graph showing an idealised waveform with its zerocrossings indicated with vertical bold lines;

FIG. 5 is a schematic graph showing the form of an actual waveform withits zero crossings indicated with vertical bold lines;

FIG. 6 is a schematic block diagram of a cycloconverter unit accordingto an embodiment of the invention for generating EM signals;

FIG. 7 is a schematic circuit diagram of a part of the cycloconverterunit; and

FIG. 8 is a schematic graph showing the normalised form of a voltagesignal as a function of time with gating periods marked in dashed linesand zero crossings with vertical bold lines.

DETAILED DESCRIPTION

An embodiment of the invention is now described. The embodiment conformsto the general description associated with FIGS. 1 and 2. The foregoingdescription of those figures is not repeated for the sake of brevity.The following description confines itself to a description of the designof the cycloconverter unit.

FIG. 6 is a schematic block diagram of the cycloconverter unit 30according to an embodiment of the invention. Power and communicationcables 50, 92 are carried to the cycloconverter unit 30 via theumbilical connection 94 which connects to the surface vessel 14. Thecycloconverter is housed in a pressure container (not shown) in thedeep-towed vehicle 18. The waveform generated by the power supply in thesurface vessel is in this example sinusoidal with a stabilised frequencyof 256 Hz, an amplitude of 2550 V. The signal may be up to 5 Ampereswith the example power supply. A 43:1 step down transformer 98 has aplurality of tappings on its secondary windings to provide a firstsinusoidal waveform and a second sinusoidal waveform on taps 54 and 58.These waveforms are supplied to the switchable semiconductor relaybridge 104 and combined to provide the required waveform to supply to,in this case, the aft electrode connection 70 via a transmission line69. The return current from the fore electrode connection 72 is to thetransformer 98 secondary winding centre-tap 56 through a transmissionline 71. A zero crossing detector 84 samples the voltage on one of theinputs to the semiconductor relay bridge 104 and provides a signalnoting the occurrence of each zero crossing to a micro-controller 80 viaan optical isolator 82. The micro-controller 80 provides instructions tothe thyristor drivers 86 which switch semiconductor relays within thebridge circuit, which is described in more detail below.

In addition several house-keeping parameters are measured andcommunicated to the surface vessel 14 through a communications interface78; the current supplied to the antenna is sampled a by current sensor74; the temperature of the apparatus is sampled by one or moretemperature monitors 76; and the voltages of the input waveforms to thebridge are sampled by voltage sensors 88, 89.

FIG. 7 is a schematic circuit diagram of the cycloconverter unit 30.Input power from the surface vessel 14 is supplied via the umbilical 16to the primary side 50 of the step-down transformer 52. A centre-tap 56on the secondary windings on the transformer 50 is provided, which maybe considered to represent a floating ground reference voltage. A firsttap 54 and a second tap 58 from the transformer 50 secondary windingsare also provided. Each of the taps 54, 58 is presented to one arm of aswitchable semiconductor bridge 104 comprising four semiconductor relays61, 62, 63, 64 arranged to form two arms as indicated in the figure. Theswitching of these relays 61, 62, 63, 64 is governed by the controller80. The outputs of each arm of the switchable semiconductor bridge 104are commonly connected to one side 69 of the signal load. The returnside of the signal load is connected directly to the centre tap 56 ofthe transformer's secondary windings through the connection line 71. Thesignal load is the resistance presented by the seawater between the foreand aft electrodes.

The generation of the output waveform from the input waveforms isachieved by controlled the switching of the semiconductor relays 61, 62,63, 64 within the switchable semiconductor bridge 104 by the controller80. Between the times t₀ and t₁ (see FIG. 3), semiconductor relay 63 andsemiconductor relay 64 are held open-circuit whilst semiconductor relay61 and semiconductor relay 62 are closed and behave as diodes. In thisarrangement, the two arms of the switchable semiconductor bridge 104alternately provide the positive going half cycles of their inputwaveforms to the switchable semiconductor bridge 104, with a commonreturn path through the load and centre-tap 56. Between the times t₁ andt₂ indicated in FIG. 3, semiconductor relay 61 and semiconductor relay62 are held open-circuit whilst semiconductor relay 63 and semiconductorrelay 64 are closed and behave as diodes. In this arrangement, the twoarms of the switchable semiconductor bridge 104 alternately provide thenegative going half cycles of their sinusoidal input waveforms to theswitchable semiconductor bridge 104, with a common return path throughthe load and centre-tap 56. Thus with appropriate switching, the outputsignal can consist of any desired sequence of half-sinusoids of thesupply frequency, and of either polarity. In typical applications, apseudo-square wave is generated by switching equal numbers of positiveand negative half-cycles in sequence, as indicated in FIG. 3.

The digital micro-controller 80 discriminates between allowable andnon-allowable pulses from the zero-crossing detector 84, and hencecontrols reliably the polarity of each output half-sinusoid of the powersupply fundamental frequency to generate the desired wave form.

The micro-controller 80 uses a timing algorithm to discriminate betweenzero-crossing pulses. The time between one acceptable zero-crossingpulse and the gating algorithm allowing the next zero-crossing to beaccepted is slightly shorter than the predicted half-period of thefundamental, by an amount which has been tuned to provide optimalperformance. Even though on start-up the cycloconverter may accept aspurious zero crossing in the first instance, it will within a fewcycles lock in to the zero crossings corresponding to the fundamentalfrequency of the supplied power. As an example, the times during which azero crossing can be accepted and acted upon according to theconstraints which might be imposed by the micro-controller 80

FIG. 8 is a reproduction of FIG. 4 with the gating periods superimposedwith dashed lines. The gating action provided by the micro-controller 80thus ensures that out-of-time zero-crossing signals are ignored. Onlythose zero crossings which occur within the gating intervals will beaccepted.

Two important benefits accrue from this arrangement. Firstly, sincewithin a few cycles of start-up the output wave form locks in to thefundamental power supply frequency and phase, unwanted frequency jitterand phase drift in the transmitted geophysical signal are eliminated.The result is a transmitted geophysical signal consisting of discretespectral lines of predictable frequency, bandwidth and peak amplitude,rather than a set of spectral frequency bands with poorly knowncharacteristics. Secondly, since the timing system which controls theoutput signal at the deep tow is effectively slaved to the fundamentalpower supply frequency, the advantages of controlling the outputfrequency and phase drift from a frequency standard on the towing vesselare obtained. This allows the use either of a self contained, highquality frequency standard such as an oven-controlled crystal, or of abroadcast frequency standard such as a GPS signal, as the controllingstandard for the stability of the transmitted geophysical signal.

1. A cycloconverter for subsea electromagnetic exploration, comprising: a transformer having a primary side for receiving a high voltage low current signal of a first frequency and a secondary side for outputting a low voltage high current signal also of the first frequency; a switching circuit connected to receive the low voltage high current signal from the secondary side of the transformer and to switch its polarity so as to generate an output signal having a significant component at a second frequency lower than the first frequency; a controller configured to control the switching circuit responsive to a measurement of the first frequency; and a zero-crossing detector for measuring zero crossings at the first frequency and supplying a zero-crossing signal to the controller, the controller being configured to supply a switching signal to the switching circuit when a zero-crossing signal is received during a time window that has been determined by the controller responsive to the measurement of the first frequency.
 2. A submersible vehicle fitted with a cycloconverter according to claim
 1. 3. The cycloconverter of claim 1, wherein the second frequency is other than a zero frequency.
 4. A method of controlling switching events in a cycloconverter during subsea electromagnetic exploration, comprising: providing a high voltage low current signal of a first frequency; transforming the high voltage low current signal of a first frequency to a low voltage high current signal also of the first frequency; measuring the first frequency; switching the polarity of the low voltage high current signal based on the measurement of the first frequency to generate an output signal having a significant component at a second frequency lower than the first frequency; detecting zero crossings at the first frequency and generating corresponding zero-crossing signals; determining a time window based on the measurement of the first frequency; and supplying a switching signal for switching the polarity of the low voltage high current signal based on a comparison of the zero-crossing signals with the time window.
 5. The method of claim 4, wherein the second frequency is other than a zero frequency. 